1. Field of Invention
The present invention relates to a method for making multilayer electrical components and more particularly to a method of making multilayer capacitors.
2. Description of Prior Art
Multilayer electrical components are presently fabricated according to three principal processes. A first process involves mixing compositions of selected ceramic dielectrics with selected dopants and/or sintering flux components to control the distribution of the ceramic components within the mixture prior to thermal and/or pressure sintering. With this process, the mixture control conditions are critical to maintain homogeneous distribution of the ceramic components and to control the sintering with reactivity promoters and inhibitors during the fabrication process.
A second process involves generating photomasks by photolithography in which a photosensitive emulsion or metal coating is deposited on a dielectric substrate in a pattern forming one or more electric circuits and then the emulsion or metal coating is selectively etched to form ceramic/metal layer. Two or more of these ceramic/metal layers are then bonded together to form a desired electrical component. Bonding of the masks is typically performed with thermal and/or pressure techniques and registration of the masks is critical to performance of the component. The process is generally very complex, time consuming and expensive according to the pattern complexity and number of the masks.
The third principal process involves preparing a ceramic dielectric layer using a doctor blade, roll compaction or similar technique and is conventionally known as tape casting. The process typically involves mixing a ceramic dielectric, especially particulates, with liquid organic binders and painting the mixture onto a flat drying surface. When dry, the mixture is peeled off as thin flexible tapes which are then covered with a pattern of particulate metal electrodes, typically by screen printing. The electrodes are printed on the tapes such that when the tapes are overlaid the electrodes of overlapping layers are in staggered formation. When the multilayer tapes are subsequently cut into pieces to form discrete multilayer electrical components, adjacent electrodes overlap internal to the component but have edges protruding from alternate sides of the component. The green, laminated component is then carefully sintered by thermal and/or pressure techniques to densify both the ceramic dielectric and the metallic electrodes. Additional external electrodes are then painted on the protruding internal electrodes. Wires are then attached to the external electrodes to form the finished component. With the last described process, because of the generally high sintering temperature needed by the ceramic dielectric, the metal electrodes must typically be high-Pd alloys to prevent volatilization of the Ag. Although ceramic dielectrics are available that are compatible with Ag-rich electrodes and that may be sintered at lower temperatures, such dielectrics have less desirable electrical properties. An undesirable choice in electrical properties of the finished component is thus presented because the cost of the Pd in the component can represent as much as half the cost of the finished component. Additionally, conventional sintering of some laminated structures such as multilayer substrates and packages typically involves sintering in belt furnaces which imposes an upper limit on structure size since the leading edge of the structure begins to shrink, leading to warping or fracture of the component, if the laminated structure is too large.
Application of microwave radiation to materials is becoming increasingly important in this last process as a tool for more efficient and effective sintering of several classes of materials. For example, U.S. Pat. No. 4,880,578 (Nov. 14, 1989); U.S. Pat. No. 4,810,846 (Mar. 7, 1989); and pending applications Ser. No. 07/781,781 and Ser. No. 07/853,474 describe use of such microwave radiation. The above-mentioned patents with issue dates in parentheses and the cited patent applications are all assigned to the same assignee as the present application and are incorporated herein by reference.
However, application of microwave radiation to multilayer electrical component structures having combined metallization and dielectric layers has been considered impractical because of arcing between the metallic layers and reaction between the materials and/or the furnace. Avoiding this limitation would enhance applications of microwave radiation to dielectric/metal composites such as multilayer capacitors, transducers, substrates, semiconductor chip carriers and the like.
The present invention avoids the limitations of the above described processes by a novel sintering process resulting in multilayer electrical components, especially capacitors, that possess the desirable dielectric properties of materials fired at high temperatures and the lower cost of Ag-rich electrodes.